Whether you're pouring a simple base for a garden shed or erecting structural foundations for a commercial warehouse, choosing the correct concrete strength is vital. Using a mix that is too weak will lead to structural cracking and subsidence, while paying for a mix that is over-specified unnecessarily inflates your project costs.
In this guide, we break down standard British concrete strength grades—categorized into GEN (General) and C (Designated/Strength) mixes—and explain which grade to select for common residential and commercial applications.
What Do Concrete Grades Actually Mean?
Concrete grades represent the compressive strength of the material, measured in Newtons per square millimeter (N/mm²) or Mega-Pascals (MPa). This value is calculated after the concrete has cured for 28 days under laboratory conditions.
For example, a C30 mix has a compressive strength rating of 30 N/mm² at 28 days. A GEN3 mix is equivalent to a C20 grade, offering a compressive strength of 20 N/mm².
"Compressive strength is the metric that dictates how much load your slab or footing can support without cracking. Matching the grade to the specific load requirements is key to longevity."
The GEN Mix Series (General Purpose Concrete)
GEN mixes are standard, lightweight, and versatile concrete grades used primarily in domestic, non-structural, or light structural building work. They are easy to level and work with.
GEN1 (10 N/mm²)
GEN1 is a low-strength mix ideal for unreinforced, non-structural tasks. It is perfect for:
- Domestic garden paths and footpaths.
- Shed bases and kennel floors.
- Backfilling trenches and drainage channels.
GEN2 (15 N/mm²)
GEN2 is slightly stronger than GEN1, designed for lightweight domestic flooring and sub-bases. Common uses include:
- Screeding preparation for floors.
- Sub-bases for patios and block paving.
- Concrete fill under conservatory floor slabs.
GEN3 / C20 (20 N/mm²)
GEN3 (often batched as a C20 mix) is the most popular grade for residential expansions and landscaping. It provides excellent moisture resistance and load bearing capacity for:
- Domestic extensions, garden walls, and structural footings.
- Garage floor slabs (without heavy vehicle parking).
- Paving bases and patio slabs.
The "C" Mix Series (Designated Structural Concrete)
The "C" (designated) series concrete is formulated for heavy-load structures, structural reinforcements, and commercial environments. These mixes feature strict cement content and water-cement ratio control.
| Concrete Grade | Strength (28 Days) | Common Applications |
|---|---|---|
| C25 | 25 N/mm² | Structural footings, retaining walls, trench fills, fence post reinforcement. |
| C30 | 30 N/mm² | Reinforced driveways, garage floor slabs (for SUVs/vans), external paving. |
| C35 | 35 N/mm² | Commercial warehouse floors, agricultural slabs, piling, structural beams. |
| C40 | 40 N/mm² | Industrial yards, data centers, agricultural roads, heavy machinery bases. |
C25 (25 N/mm²)
C25 is a structural grade mix suitable for general groundworks and domestic foundations. It offers the structural strength required to pass local building control checks for extension footings in clay soil regions.
C30 (30 N/mm²)
C30 is designed to withstand freezing, thawing, and heavy abrasion. If you are paving a driveway that will park heavy vehicles (like SUVs, vans, or delivery trucks), C30 is the minimum specification to prevent cracking. We often add reinforcing fibers directly to C30 mixes on-site to increase flexural strength.
C35 & C40 (35-40 N/mm²)
These are high-performance commercial and industrial-grade concrete mixes. They offer extreme resistance to acidic environments, heavy machinery loads, and chemical exposure, making them ideal for warehouse loading docks, farm yards, and structural underpinning.
Why Volumetric Batching is Ideal for Concrete Grades
If you order concrete from a traditional batching plant and realize your trench is deeper than anticipated, you're stuck with whatever mix you ordered. With a volumetric mixer from Total Mix Concrete, the operator can adjust the mix design instantly on-site. If you need a C25 mix for extension footings followed by a GEN1 mix for a fence wall, we can change the mix type on the fly, saving you from ordering two separate deliveries.
How to Order the Right Mix
Still unsure which grade you need? Consult your structural engineer's drawings. If you're a homeowner planning a DIY patio, driveway, or conservatory, call us. Our team regularly delivers ready mix concrete in Watford and across London, providing expert guidance on structural specs and assisting with our volumetric mixers.
Calculate Your Required Concrete Volume
Use our interactive concrete volume calculator to find out exactly how many cubic meters you need. Order via our volumetric mixers and pay only for what you pour—with zero waste.